The present invention relates to a metalworking method and to the corresponding product obtained with the method, particularly lengths of tube for various uses made of steel having a carbon content of 0.10% to 0.50%, i.e., steel of the type ranging from AISI/SAE 1010 to AISI/SAE 4150 (special casehardening steels and hardening and tempering steels from C10 to 50CrMo4), as listed for example in the tables of "The Stahlschlussel Reference Book (Key To Steel)", 1989.
Currently, the most widely used methods for obtaining lengths of tube made of steel of the above-cited type are forging, hot extrusion (also known as drawing), forward cold extrusion, hot rolling (seamless, method known as Mannesmann method), and drilling from a length of rolled solid bar.
As regards the forging process, it entails the following steps (see FIGS. 1, 2 and 3): one begins with a length 1 of round bar or billet of suitable size (for example a billet with a side A having a square cross-section of 40 to 140 mm), cut to a length which depends on the volume of the part to be obtained (see FIG. 2).
The billet length 1 is heated to a temperature of approximately 1200.degree. C. in a furnace, usually of the gas-fired type.
Punching is then performed: the still-hot length of billet 1 is placed on top of a die 2 arranged inside a press and upsetting with a punch 3 is performed until a typical cup-like shaped part 4 (see FIGS. 1 and 3), with an axial cavity having a diameter a, is formed.
The shaped part 4 is then extracted and is placed, while it is still hot, inside a cylindrical die 5 and then deformed by means of a punch.
The metallic material is compressed with a given force F by the punch 3 and assumes the profile of the punch and of the die, obtaining a rough-shaped tube 6 of suitable thickness.
The tip 7 of the tube (also known as bottom) that remains to be shaped is then trimmed or cut (see FIG. 4).
The last step of the process is the cutting of the tube 6 to obtain lengths of the intended size.
Forging is usually performed in a plurality of passes until the intended shape of the tube is obtained.
This known process therefore allows to obtain a rough-shaped tube whose length, besides depending on the thickness, can exceed one meter only with great difficulty; moreover, the external and internal finish has scale, scores and other imperfections; concentricity tolerances which can reach 10 to 30% of the thickness of the tube are also obtained.
The described process also entails other drawbacks, since very large and expensive machines are required.
Moreover, since tool changing processes are time-consuming, this known process requires minimum quantities of product in excess of 100 tons in order to be financially convenient.
Finally, this process is not suitable for producing low-thickness tubes.
The product obtained with the method further requires additional working in order to obtain a length of tube suitable for the above-described uses, such as internal and external turning in order to restore the necessary tolerances and eliminate the layer of material whose metallurgical characteristics have been altered by heating.
Finally, it is necessary to cut the bars in order to obtain a length of tube having the intended longitudinal dimension; approximately 2 to 5% of the material is wasted in the trimming step for this treatment.
The hot extrusion process instead entails the following steps. Up to the punching step, the process is identical to the forging process. The part is then extracted while still hot, and external and internal drawing is performed; during the drawing, the metallic material, pulled by a force F through a tool known as drawplate, assumes the profile of the drawplate, with a deformed cross-section which is smaller than the cross-section it had at the inlet, and the thickness of the tube is also reduced by means of a mandrel or punch 8 inserted internally (see FIG. 5).
The bottom is then trimmed or cut as in the previous method.
The last step of the process consists in cutting the tube to obtain lengths having the chosen longitudinal dimension.
Drawing is usually performed in a plurality of passes which depend on the thickness of the tube to be obtained.
This known process can be used on a smaller range of steels and the resulting product is constituted by a rough-shaped tube whose length, besides depending on the thickness, can exceed 2 meters with great difficulty, with an internal and external surface finish having scale, scores and other imperfections, and with concentricity tolerances which can reach 10 to 30% of the thickness.
The process differs from hot forging in that it is more suitable for tubes of considerable length and for low thicknesses.
Moreover, the described process entails other drawbacks, since it is necessary to use very large and expensive machines.
Tool changing processes are still time-consuming and accordingly the process still requires minimum product quantities in excess of 100 tons in order to be financially convenient.
The resulting product requires further working in order to obtain a length of tube which is suitable for the above-described uses, such as internal and external turning in order to restore the necessary tolerances and eliminate the layer of material whose metallurgical characteristics have been altered by heating, and finally requires the cutting of the bars in order to obtain the length of tube having the intended longitudinal dimensions.
Approximately 2 to 5% of the material is wasted in the trimming step for this known type of metalworking.
We now consider the known forward cold extrusion process: it entails the following steps (shown sequentially in FIG. 6):
the cutting of a length 10 from a round bar of rolled steel (the length has a longitudinal dimension which depends on the dimensions of the part to be obtained); PA1 sanding in order to eliminate steel rolling scale and to prepare the surface of the part; PA1 chemical surface treatment of the material by means of cleaning--phosphatizing--neutralizing--stearate treatment (or soap treatment) steps; PA1 punching of the material: the block is pressed in a press (with a rating of at least 200 tons) to obtain a first cup-shaped part 11. PA1 producing a round bar of hot-rolled steel; PA1 peeling said bar; PA1 cutting said bar so as to obtain at least one block; PA1 drilling said block; PA1 chemically treating said drilled block; PA1 pressing said block.
The part is again subjected to a chemical surface treatment similar to the previous one.
The part is then extruded (the material, arranged inside a die, due to the pressure applied by the punch, causes the extruded element to flow in the opposite direction with respect to the advancement of the punch) and is thus elongated to the required size (reference numeral 12).
A new step of chemical surface treatment is then performed.
Finally, the end (or bottom) 13 is trimmed mechanically.
This process can be used only with steels of the type having a carbon content up to 0.20% (AISI/SAE 1020 steels) and with rolled round bars having a diameter of less than 60 mm.
The finished product is constituted by a tube 14 having a good internal and external finish, with size tolerances within 0.20 mm and with concentricity values variable from 0.4 to 0.5 mm.
However, in order to obtain tubes having external dimensions of more than 60 mm (again for steels with a carbon content up to 0.20%), it would be necessary to provide additional extrusion steps, and preliminary annealing, sanding and chemical surface treatment operations would be required before each extrusion step and also before the trimming of the end.
Extrusion for materials with a carbon content above 0.20% is possible but requires a normalization (or spheroidizing) step to optimize the formability of the steel after each one of the cutting, punching and extrusion steps and after each additional extrusion step. Accordingly, the advantages that can be achieved with the process are obtained only in large-volume production (above 50 to 100 tons of product) and with plants of considerable size and power.
This process, too, requires large and expensive machines with very long tooling times which accordingly require very large production batches (50 to 100 tons of product).
Costs increase due to the large number of formability optimizing treatments for steels having a carbon content in excess of 0.20% or with a diameter of more than 60 mm.
There are additional very high costs for the manual treatment of the material during the phosphating and stearate treatment step; the cup-like shape in fact entails the risk that the part might contain the liquid when it leaves the chemical treatment and the part must therefore be emptied.
Approximately 10% of the material is lost during trimming for this kind of metalworking.
We now consider the hot-rolling (seamless) process.
The hot-rolling process is universally known and designated as the Mannesmann process; its description is omitted because it is known (an exemplifying diagram is provided in FIG. 7).
This process can be used on a great variety of materials: the product that is obtained is a rough-shaped tube with a length of even 6 to 8 meters, with an external and internal surface finish which is better than in the previously described hot processes, but nonetheless with a production of scale, scores and other imperfections and with concentricity tolerances which can reach 0.7 to 1.0 mm.
The process differs from other hot processes in that it is more suitable for tubes of considerable length and with low thicknesses.
The described process also entails other drawbacks: the processes for changing the tools are time-consuming and therefore the process again requires minimum product quantities in excess of 100 tons to be financially convenient.
Very large and expensive machines are again required and the resulting product requires further working in order to obtain a length of tube which is suitable for the above-described uses, such as internal and external turning in order to restore the necessary tolerances and eliminate the layer of material whose metallurgical characteristics have been altered by heating, and finally requires the cutting of the bars in order to obtain the length of tube having the intended longitudinal dimensions.
Finally, we consider drilling from a length of rolled solid bar.
This method entails the following mechanical treatments: the cutting of an initial block from rolled round steel bars; and mechanical chip-forming machining of the block at its inside and outside diameters and along its length.
This known process can be used in various materials and allows to obtain a product constituted by a tube having a good internal and external finish which meets the required tolerances; however, there are drawbacks, such as long treatment times, high tool wear, especially with materials having a carbon content of less than 0.20%; moreover, for these materials, owing to their limited chip-forming ability, the possibility to machine the bore with points of the hard-metal type is limited, consequently increasing the machining times and the costs; a high consumption of necessary material is also observed since more than 50% of the material is lost as machining waste.